A booster refers to a device that boosts a pedal effort of a driver, which is transmitted from a brake pedal to a master cylinder. An electric booster refers to a device that interprets a pedal effort of a driver as an electrical signal, drives a motor based on the interpreted electrical signal, and thus produces a brake pressure by pressing the master cylinder. As electric vehicles and hybrid vehicles become prevalent, interest has increased in recent years about the electric booster as a device that is substituted for a hydraulic booster.
A process of producing a brake pressure by the electric booster will be concisely introduced with reference to FIG. 1.
FIG. 1 is a view illustrating a brake system including a typical electric booster.
Referring to FIG. 1, a pedal effort of a driver is transmitted to a sub master cylinder 2 when the driver steps on a pedal 1. A solenoid valve 3 connected to the sub master cylinder 2 forms a hydraulic line P1 from the sub master cylinder 2 to a pedal simulator 4. The pedal simulator 4 provides a reaction force for the pedal 1 to make the driver sense the pedal effort. An oil reservoir 9 is connected to a master cylinder 8.
Meanwhile, a stroke sensor 5 is mounted at the pedal 1 to detect a stroke by a rotational angle of the pedal 1. An ECU 6 recognizes a required brake force of the driver based on the detected stroke, and drives a motor 7 based on the required brake force, and thereby the master cylinder 8 is pressed to produce a brake pressure. Here, the ECU 6 appropriately feedback controls the brake pressure through pressure sensors 10 installed at the sub master cylinder 2 and the master cylinder 8.
In this electric booster, a highly reliable operation of the motor is a very important factor. However, apart from a control error of the motor, a physically locked state of the motor may occur. For example, a locked state in which the motor is not moved may occur due to mechanical or electrical defects of the motor, eccentricity of the motor, damage to a ball screw connected to the motor, or the like. In a case in which the brake pressure is additionally applied in the locked state of the motor, fatal problem with brake safety may occur.